In recent years, various techniques for detecting an occupant in a seat of a vehicle has been proposed. That is, an occupant detection system is used mainly to detect respective occupants in driver's and passenger's seat in a front row of the vehicle for use with devices such as an air-bag system, a seat belt warning system or the like. However, a requirement for providing the occupant protection system for the occupants in rear seats is on a rise, and that leads to a demand for detecting more occupants in the vehicle. Therefore, a conventional occupant detection system disclosed in, for example, Japanese patent documents JP-A-H10-39045 and JP-A-2005-153556 (latter one is published as US publication document No. 20050140210) and other reference for use in the passenger's seat is also disposed on the rear seat for achieving the goal of rear seat occupant detection.
In this case, a control unit in the occupant protection system detects the occupancy of each of the seats based on occupant sensors disposed on each of the seats. However, according to the configuration of the occupant protection system disclosed in the above documents, input terminals on the control unit for connection to the occupant sensors increases in number.